The present invention generally relates to a system for supporting secure communication of information and more specifically to a system for conducting the U.S. Government's Secure Terminal Unit-III (STU-III) and the NATO version STU-IIb communications over packet networks.
For certain applications, it is necessary to be able to establish secure communications between multiple terminals. The U.S. Government's Secure Terminal Unit-III (STU-III) and the NATO version STU-IIb are examples of such systems. The secure communications are typically realized by the use of encryption technology within the terminals. In a standard network connection, two terminals are connected together across a telephone network. Typically, the telephone network is digital and converts the analog transmissions from the terminal to a digital stream at, e.g., 64 kb/s using Pulse Code Modulation (PCM) techniques for the digitization. Once a connection is established between the two terminals, the call switches from a regular voice call to a modem (modulator-demodulator) call. Basically, STU works by establishing a special modem call between the terminals. Over the modem call, the encrypted voice communications are sent. The analog modem signal is captured, digitized at the start of the digital circuit using PCM, and within certain parameters, faithfully reproduced at the other end.
A technique has been previously introduced to reduce the amount of bandwidth required in the digital network to carry the STU call. This technique, called a STU relay, demodulates the STU call that entered the digital network, and only transmits the baseband data. At the other end of the network, the baseband data is re-modulated so as to transmit the expected signal to the other terminal. This demodulation and re-modulation by the relay is transparent to the terminals. This approach reduces the required bandwidth to transmit the baseband data to between 2.4 and 9.6 kb/s, which is considerably less that the regular 64 kb/s bandwidth used for calls that are not demodulated. Hence the value of the STU relay.
STU relay communications transfer data in a synchronous data stream. Thus, STU relay communications have been limited to synchronous networks, such as public switched telephone networks (PSTNs) and low-rate digital networks. In a synchronous network, data, whether it be idle bits or data, have to be sent at all times through a dedicated connection. Also, data is received exactly as it is transmitted. Thus, if data is sent from time, t=0 to t=100, idle bits from t=100 to t=500, and data from t=500 to t=600, the data and idle bits will be received by a receiver with the same time intervals although there may be a delay for transmitting the data from the transmitter to the receiver. Accordingly, if the first bit of data is received at a t=1000, the receiver receives data from t=1000 to t=1100, idle bits from t=1100 to t=1500, and data from t=1500 to t=1600. From the above, the data is received exactly as transmitted and the gap where idle bits were sent is the same as transmitted. Thus, STU relay communications have the advantage that data is received as it is sent; however, the communications are limited to synchronous networks.
Accordingly, there is a desire for developing a system for supporting secure transmission of information over asynchronous networks, such as packet-based networks.